Thermodynamic Analysis and Screening ILs/DESs-based Absorbents for CO2 Separation

Sammanfattning: CO2 separation plays an important role in both biofuel production, and CO2 capture and storage (CCS) implementation to deal with global warming. The available CO2 separation technologies are either energy-intensive or require large-scale operations, and it is crucial to develop novel CO2 separation technology in order to optimize the energy uses and the amounts of CO2-absorbents/adsorbents.Recently, ionic liquids (ILs) have been proposed as potential liquid absorbents for CO2 separation with remarkable properties. A lot of ILs have been synthesized for this purpose. The CO2 absorption capacity/selectivity and the energy use have been considered in screening ILs, while the amounts of ILs needed have seldom been considered in the screening process. Meanwhile, the high-cost, toxicity and poor biodegradability of the conventional ILs limit their applications in large-scale. Deep eutectic solvents (DESs) have emerged as a new type of ILs, and in particular, those based on choline salts (i.e. choline-based DESs) show additional advantages in cost, environmental impact and synthesis. Choline-based DESs have been synthesized and the research work related to CO2 separation with this series of DESs and their aqueous solutions has been carried out. However, it is still unclear which absorbent can achieve a better performance for CO2 separation.The choice of absorbents for CO2 separation depends on gas streams, and the performances of absorbents for CO2 separation relate to the energy uses and the amounts of absorbents needed. In this thesis work, four gas streams (i.e. flue gas and lime kiln gas from the combustion of fossil-fuels, biogas from the anaerobic digestion of biomass as well as bio-syngas from the gasification of biomass) with different temperature, pressure, CO2 concentration and gaseous components were considered, and CO2 separation from four gas streams was analyzed thermodynamically based on Gibbs free energy change. The analysis shows that biogas is the CO2 stream with the lowest theoretical energy penalty. Therefore, biogas was chosen as a specific CO2 stream for further evaluating the performances of CO2 absorbents.In evaluation, the conventional ILs were first analyzed and screened for CO2 separation from biogas with three options (i.e. option 1: the CO2 dissolution enthalpy and CO2 working capacity, option 2: the energy use, and option 3: the energy use and the amount of IL needed). The investigation shows that the screen of ILs is strongly related to the operational condition and the screening criteria. In the option of “the energy use and the amount of IL needed”, the operational condition was optimized based on the minimum Gibbs free energy change, and the energy use and the amount of IL needed were considered in screening. While in other screening options, the operational conditions were presumed and the amounts of ILs needed were not considered. Therefore, the option of “the energy use and the amount of IL needed” is more reasonable compared to the other two options. The performances of these screened conventional ILs were further compared with those of the commercial CO2 absorbents. It shows that the conventional ILs are promising CO2 absorbents due to lower energy uses or lower amounts of ILs needed combined with the advantage of non-volatility.The research work on choline-based DESs and their aqueous solutions for CO2 separation was surveyed and reviewed. Generally, the properties of choline-based DESs are similar to those of conventional ILs. Considering the additional advantages of low-cost, non-toxicity and biodegradability, choline-based DESs are more promising for CO2 separation. However, due to the limited available research work, further studies need to be carried out from experimental measurements to model developments. The performances of choline-based-DESs for CO2 separation from biogas were analyzed. Based on the option of “the energy use and the amount of absorbent needed”, the choline-based-DESs were screened and then compared with the conventional ILs and the commercial CO2 absorbents. The comparison results show that the choline-based-DESs are more promising for CO2 separation from biogas due to the non-volatility, lower energy uses or lower amounts of absorbents needed. In addition, CO2 separation from other CO2 streams was further investigated. It shows that the physical absorbents are more suitable for the CO2 streams with high CO2 concentration (i.e. biogas, lime kiln gas and bio-syngas), while the chemical CO2 absorbents are more suitable for that with low CO2 concentration and high temperature (i.e. flue gas). Considering the high amounts of physical absorbents, further study needs to be carried out with techno-economic analysis.